Real Dictionary

gravitational lens in American English
a massive celestial body, esp. a galaxy or cluster of galaxies, having enough gravity to refract light waves from a more distant object, esp. a quasar, so that an observer sees an amplified or multiple image (gravitational lensing)
noun: a heavy, dense body, as a galaxy, that lies along our line of sight to a more distant object, as a quasar, and whose gravitational field refracts the light of that object, splitting it into multiple images as seen from the earth

gravitational lens in British English
noun: a lenslike effect in which light rays are bent when passing through the gravitational field of such massive objects as galaxies or black holes